It is known in the prior art to mount semi-conductors to heat sinks which serve to dissipate heat from the semi-conductors during operation. It is important that the semi-conductors be firmly attached to a heat sink while at the same time the means for attaching the semi-conductors to the heat sink must be removable in the event that the semi-conductors must be replaced.
It is, therefore, important to provide removable semi-conductor clamping means so that the semi-conductor may be removed and replaced with a new semi-conductor.
One means known in the art for clamping or securing one or more semiconductors to a heat sink is to place an aluminum bar over the top of one or more semi-conductors and then secure the aluminum bar to the heat sink by use of a fastening means, taking the form of a screw. The screw extends through a hole in the aluminum bar and is threaded into the heat sink by applying torque to the screw. Several problems have been noted with the use of such a clamping mechanism. If the top and bottom surfaces of the semi-conductor device or package are not parallel to each other, then an uneven distribution of pressure will result between the clamp bar and the semi-conductor and between the semi-conductor and the heat sink. This may lead to a cracked semi-conductor housing, and/or less than optimal thermal performance. This problem becomes more exaggerated in situations where multiple semi-conductor packages are held down by such a clamping mechanism and wherein the semiconductor packages are of varying heights and, hence, parallelism is not obtained with a single clamp bar extending over a plurality of such semi-conductor packages.
Another problem encountered with such clamping mechanisms as noted above is the unpredictable and unrepeatable torques exerted by the screw employed to hold the overlying clamp bar down against the upper surfaces of the semi-conductor packages. Thus, since the pressure exerted by the clamping mechanism to hold the semi-conductor packages down against the heat sink depends in large measure on the torque applied to the screw, different semi-conductors held down by different clamping mechanisms constructed in the same manner will provide different levels of hold-down pressure, since each clamping mechanism will exert a hold-down pressure dependent upon the torque applied to its screw. This will result in different levels of hold-down pressures for different semi-conductors mounted to the same or different heat sinks.
The U.S. Patent to Ewer et al. U.S. Pat. No. 4,853,762 discloses spring clamps for resiliently clamping a pair of semi-conductor devices to a support, such as a heat sink. A compression disk overlies each semi-conductor device and has a dome-shaped upper surface against which a spring exerts downwardly extending forces so that the compression disk, in turn, holds the semi-conductor devices against the heat sink. Each spring overlies and contacts one compression disk and each spring is secured to the heat sink by means of two screws which extend down into and are threaded into the heat sink.
The clamping mechanism disclosed in the Ewer et al. patent, supra, depends on the height of the screws to determine the amount by which the associated spring is deflected. Depending upon the amount of torque applied, the screws may be over or under tightened. Moreover, each overlying spring in the Ewer et al. patent is deflected in a downward direction raising the possibility that the spring may interfere with other devices and may interfere with electrical connections and the like. Moreover, it is noted that Ewer et al. requires two springs to hold down two semi-conductor devices and each spring requires two screws to secure the spring to the heat sink. It would be more desirable to employ a single spring to hold down two devices wherein the spring bends upwardly to minimize interference with electrical connections or other devices and wherein a spacer or the like is provided to prevent overtorqueing of the screws to thereby avoid damaging the semi-conductor packages.
It is further noted that each compression disk employed in the Ewer et al. patent, supra, has a flat lower surface and rests on top of a flat upper surface of a semi-conductor device thereby distributing forces evenly to the top of the semi-conductor device on which it is placed. If such compression disks are placed over a conventional "pill" type semi-conductor package which takes the form of upper and lower flat relatively parallel circular-shaped surfaces interconnected by a cylindrical wall, such forces distributed by the compression disk may cause the central portion of the upper surface of the semi-conductor packages to be damaged and, possibly, collapse. Moreover, the use of such compression disks having a flat lower surface provides no means for locating and retaining the semi-conductor "pill" package during assembly.